The phosphorylation of tyrosine residues in eukaryotic proteins has been shown to play extremely important roles in the regulation of numerous eukaryotic cellular processes (Fantl et al., Annu. Rev. Biochem. 62:453-481 (1993) and Hunter, 1001 Protein Kinases Redux Toward 2000 5:367-376 (1994)). While a great deal of information has been accumulated regarding the functions of the protein tyrosine kinases, far less is understood about the physiological roles of protein tyrosine phosphatases (PTPs), the enzymes which remove phosphate from tyrosine residues in proteins. While approximately 50 PTPs have now been described, the functions of only a very few are beginning to be understood (Tonks, Semin. Cell Biol. 4:373-453(1993) and Dixon, Recent Prog. Horm. Res. 51:405-414 (1996)). However, in general, it appears that many of the PTPs function to modulate the positive or negative signals induced by various protein tyrosine kinases. Therefore, it is likely that PTPs play critical roles in numerous and diverse cellular processes.
The PEST family of PTPs are a group of phosphatase enzymes. The four known examples of these enzymes, PTP PEST [Yang et al., J. Biol. Chem. 268(23): 17650 (1993)], PTP PEP [Matthews et al., Mol. Cell. Biol. 12(5):2396-2405 (1992)], PTP HSCF [Cheng et al., Blood 88(4):1156-1167 (1996); U.S. Ser. No. 08/620,526 filed Mar. 22, 1996]; also known as PTP-K1 [Huang et al., Oncogene 13:1567-1573 (1996)], PTP20 [Aoki et al., J. Biol. Chem. 271(46):29422-29426(1996)] or FLP1 [Dosil et al., Blood 88(12):4510-4525 (1996)] and PTP BDP1 (Kim et al., Oncogene 13:2275-2279 (1996)), all contain an N-terminal phosphatase domain which is followed by a variably sized region that is rich in proline, serine and threonine residues, but which has no obvious homology to other proteins. The PEST family of PTPs also contain a highly conserved 20 amino acid long proline rich region at the very C-terminus of the proteins which are believed to be involved in protein-protein interactions. With regard to cell type expression, PTP PEST is ubiquitously expressed (Yang et al., (1993) supra), PTP PEP is expressed in lymphoid cells (Matthews et al., (1992) supra), PTP HSCF is expressed in hematopoietic stem/progenitor cells and fetal thymus (Cheng et al. (1996) supra and Dosil et al., (1996) supra) as well as a subset of adult tissues including bone marrow (Huang et al., (1996) supra) and PTP BDP1 is expressed at low levels in the brain as well as other adult tissues (Kim et al., (1996) supra).
Insight into the physiological functions of PEST PTPs may be obtained from an examination of the proteins which interact with these enzymes, the effects of overexpression of the proteins on cellular differentiation and the possible modes of regulation of the molecules. Transfection of dominant negative forms of PTP PEST into COS cells results in an endogenous, hyperphosphorylated protein that has been identified as p130.sup.CAS, a cytoplasmic docking/adaptor-type molecule which contains an SH3 domain as well as several potential tyrosine phosphorylated SH2 binding sites (Garton et al., Mol. Cell. Biol. 16(11):6408-6418 (1996)). The function of p130.sup.CAS is incompletely understood, but it appears to be associated with focal adhesions and is phosphorylated by the p125.sup.FAK (Petch et al., J. Cell. Sci. 108:1371-1379 (1995) and the RAFTK (Astier et al., J. Biol. Chem. 272(1):228-232 (1997) tyrosine kinases, suggesting that it may play a role in integrin-mediated signal transduction. Because dominant negative PTP PEST inhibits dephosphorylation of p130.sup.CAS, it is likely that this phosphoprotein is a substrate for this PTP.
Interestingly, it has also been recently shown that the PTB domain of the cytoplasmic adaptor protein SHC interacts with a non-phosphorylated PTB-related binding site in the C-terminal region PTP PEST (Charest et al., J. Biol. Chem. 271(14):8424-8429 (1996)). In addition, recent data have demonstrated that Csk, a cytoplasmic tyrosine kinase which inactivates Src family kinases by phosphorylation of their C-terminal inhibitory tyrosines, associates with the PEP PTP via an interaction between the Csk SH3 domain and one of the four proline-rich potential SH3 binding sites in the C-terminal region of the enzyme (Cloutier et al., EMBO J. 15(18):4909-4918(1996)). Together, these results suggest that the biological activities of PTP PEST and PTP PEP (as well as possibly other PEST PTPs) are mediated through their interaction with critical cytoplasmic signaling proteins involved with the transmission of information from various cell surface receptors.
However, it is believed that the PSTPIP proteins which bind to and are dephosphorylated by members of the PEST-type protein tyrosine phosphatases have not been heretofore disclosed. Therefore, it is an object of the present invention to provide PSTPIP polypeptides which bind to and are dephosphorylated by members of the PEST-type protein tyrosine phosphatases.
It is a further object of the present invention to provide nucleic acid encoding the PSTPIP polypeptides so that those polypeptides may be prepared by recombinant DNA techniques.
These and further objects will be apparent to the ordinarily skilled artisan upon consideration of the specification as a whole.